A FPA assembly typically includes, in addition to the FPA itself, a dewar to maintain the FPA at a desired cryogenic operating temperature, support electronics (such as readout integrated circuits), and cabling.
One off-FPA approach to providing a temperature measuring circuit for a FPA assembly employs a conventional 2N2222-type diode. A single two wire measurement is used to measure the diode voltage at a known current. As is well known, the diode voltage is a function of the temperature of the diode junction. However, this approach has been found to have several disadvantages.
For example, the diode or diodes (often two are used for redundancy) each require from two to four leads. These leads are in addition to those required to operate and convey the output signals from the FPA assembly. This arrangement can present a problem for some applications, in that the additional diode leads adversely impact the heat load, reliability, and the cost of the FPA assembly.
Another disadvantage to this conventional off-FPA approach is that the diode(s) must be individually calibrated to account for process variations. Calibration typically involves measuring the diode voltage drop (Vdiode) at a reference current, such as 1 mA. The required calibration procedure adversely impacts the cost and processing complexity of the FPA assembly.
Another technique for measuring temperature is known as an on-FPA method. This method is presently accomplished using one of several techniques: (a) an unbiased diode, a MOSFET, or a resistor on a readout integrated circuit is accessed via two pads per temperature sensor; (b) a diode, MOSFET, or resistor is biased with an on-readout current source and is then accessed in a voltage mode via one or two pads per temperature sensor; or (c) a set of diodes, MOSFETS, or resistors, along with associated current sources and buffers, are incorporated into the video data stream that is output from the FPA.
However, each of these techniques suffer from the calibration problems described above. Furthermore, methods (a) and (b) also have the disadvantage of requiring extra leads.